This application is the national phase under 35 U.S.C. xc2xa7 371 of PCT International Application No. PCT/JP99/05628 which has an International filing date of Oct. 12, 1999, which designated the United States of America.
The present invention relates to a metal filter capable of filtering a fluid such as a molten polymer or various gases such as a gas for producing semiconductors, with high precision and good efficiency.
Conventionally, porous sintered bodies obtained by molding and sintering a powdery metal substance such as metal fibers or metal powder are, owing to the following advantages, used in numerous fields of filtration recently in particular such as for filtering a process gas for producing semiconductors, for example, as a filter capable of removing fine particles of 0.01 xcexcm or less with high precision.
1 That the filtration characteristics can be adjusted in accordance with the type of the powdery substance such as the size of the powder or fibers to be used.
2 That they have a high heat resistance and a corrosion resistance, and can be applied to numerous types of gases, and in addition can withstand a high temperature process such as a baking process.
3 That they, being excellent in mechanical processability, can be welded and post-processed, and can be easily joined and integrated with a housing or a pipe line.
4 That they have a large mechanical strength and have a high pressure resistance and toughness.
Meanwhile, in the case where a sintered body having a small pore diameter was to be produced for improvement in filtration precision in such a metal filter, the powdery substance such as metal powder or metal fibers was made finer or the thickness of the sintered body was increased in order to utilize the thickness effect of the sintered body. Such a filter gives a large pressure loss and decreases the filtration efficiency.
For this reason, the present applicant in the international patent publication WO93/06912 proposes a sintered body obtained by immersing a porous supporting member in a suspension containing a powdery substance suspended therein without using a binder and evacuating it to form a fine particle layer for sintering.
According to this proposal, since fine metal particles are molded to have a thickness as small as possible and carried on a supporting member, it is possible to obtain a filter being excellent in filtration precision and capable of restraining the pressure loss. As a result, a constant filtration process is made possible with a small filtration area by being capable of suppressing the pressure loss, thereby contributing to space saving such as scale reduction of apparatus and reduction of space between equipped pipe lines. Here, the filter can be made in various shapes and configurations such as plate-like, tubular, and cup-like, and can also be machined.
However, in filtration apparatus of recent years, a further improvement in the filtration precision and a further decrease in the pressure loss are desired.
In order to reduce the pressure loss, enlarging the filtration area is effective; and, in conventional cases also, regarding a filter composed for example of a flexible non-woven fabric or a network, a chemical synthetic fiber filter or the like which is bent in a waveform or fluted after its molding is used. However, the aforesaid metal sintered body, in particular a metal sintered body made of a laminate such as in the aforesaid proposal, is liable to generate cracks and cleavages easily by a bending process after the molding.
For this reason, in order to increase the filtration area in such a metal sintered body, it may be considered to make an uneven surface by partially grinding and pressing a filtering layer or consciously allowing the adsorption amount of the powdery substance to be non-uniform at some places at the time of suction-molding to impart an uneven surface to the filtering layer to respectively increase the filtration area. However, methods according to machining are liable to cause decrease in the filtration efficiency due to generation of grinding burr and partial high density at a processed portion to clog the pores. Also, by consciously changing the lamination thickness as in the latter case, it is difficult to form an uneven shape to such a degree that the filtration area increases, particularly in a comparatively small filter. Moreover, there is a problem that partial variation in the pressure loss increases.
The object of the present invention is to provide a metal filter being highly precise and excellent in filtration efficiency based on the idea of making the filter to have an undulating filtering-surface by allowing the covering surface of the supporting member to be uneven and covering the surface with a fine filtering layer, and of allowing the filtration layer thickness at valley portions on the undulating filtering surface to be larger than the filtration layer thickness at the summits of crest portions so that a fluid to be processed flows uniformly through the filter.
In order to achieve the aforesaid object, the present invention provides a metal filter comprising a fine filtering layer having a thickness that cannot by itself maintain its shape against filtration pressure, and a supporting member sintered to be porous and having a covering surface to integrate the filtering layer into a laminate to maintain the shape, wherein an average pore diameter of pores of said fine filtering layer is et to be smaller than an average pore diameter of pores of said supporting member, and said covering surface of said supporting member is formed in uneven shape having crest portions so as to allow a filtering surface of the fine filtering layer to be an undulating filtering surface such that an area ratio S1/S0 of its real filtration area S1 to a standard filtration area S0 of the case where it is not formed in an uneven shape, is larger than or equal to 1.3, and that an average thickness of said fine filtering layer at valley portions between said crest portions is set to be larger than an average thickness of said fine filtering layer at the crest portions, and the filtering surface of said fine filtering layer is formed in a smoothly curved surface shape.
Said supporting member may comprise a tubular body in which said crest portions are adjacent to one another in a circumferential direction and extend in an axial direction; and a pitch of the crest portions in the circumferential direction may be set to be 2 to 4 times as large as an elevation height of the crest portions from said valley portions to said crest portions. Said fine filtering layer may be a porous sintered body of a powdery substance containing metal short fibers having an average diameter d smaller than or equal to 3 xcexcm and an average of an aspect ratio L/d of length L to diameter d being 2 to 20.
Further, it may be a metal filter which filters a gas so that the number of particles having a size larger than or equal to 0.01 xcexcm is substantially smaller than or equal to 10xe2x88x9210, and in which said gas is a gas for producing semiconductors.
Thus, the metal filter can perform a high precision filtration efficiently and also is provided with a mechanical strength by forming an undulating filtering surface to make improvements in the filtration area, allowing the fine filtering layer to have such a thickness that cannot by itself withstand the filtration pressure, and allowing the maintenance of the filter shape to be carried out substantially by the supporting member.
In such a filter, the covering surface of the supporting member in the present invention is formed in an uneven shape having crest portions, and is made into an undulating filtering surface by covering the fine filtering layer along the uneven shape. The degree of undulation for this case is set to be larger than or equal to 1.3 times as large as the filtration area of the case where the undulating shape is not formed.
The reason why the filtration area ratio is set to be larger than or equal to 1.3 is as follows. If the fluid to be processed is, for example, a high-purity gas or a polymer as a target, ordinary filters having fine pores tend to increase the pressure loss to cause denaturing during the processing. In order to prevent this, there is a necessity of performing the filtration efficiently, and also there is an already-made size for filter containers. Further, if laminate structures having such different pore characteristics are integrated, for example, by sintering, it is preferable to set said filtration area ratio to be at least equal to or larger than 1.3 times in order to restrain the layer exfoliation and generation of cracks caused by difference in heat-shrinkability. However, setting the filtration ratio to be larger than necessary makes the undulation pitch rather narrow or produces large projections, so that it is difficult to form a good fine filtering layer. Therefore, it is preferably set to be smaller than or equal to 5 times, more preferably 1.5 to 3 times.
Further, in such a laminated metal filter having an uneven shape formed thereon, it is often the case that the flow resistance differs at each portion in accordance with the overall uneven thickness. Therefore, in a filter in which the unevenness is simply formed to increase the filtration area, portions having a smaller flow resistance preferentially undergo the filtration process, thereby causing retention or denaturing.
Therefore, in the present invention, the problems have been solved by setting the filtration layer thickness of at least the crest portions of the uneven shape, where the flow resistance is large, to be smaller than the filtration layer thickness at the valley portions in order to use the entire filter surface uniformly.